1. Field of the Invention
The present invention relates to the field of power plant technology. It relates to a method for increasing the efficiency of a power plant which is equipped with a gas turbine, and in particular to a method for preheating fuel. The invention also relates to a power plant for carrying out the method.
2. Description of the Related Art
By increasing the fuel temperature in a gas turbine, it is possible for the distribution of the fuel-air mixture in the burners of the gas turbine to be improved, which leads to a reduction in NOx emissions. What is more important is that, in the case of a combined cycle power plant (CCPP), the overall efficiency of the gas turbine and thus also of the combined cycle plant can be improved if the fuel preheating is integrated into the water/steam circuit of the combined cycle plant. In the case of a combined cycle plant of type KA26-1 (with gas turbines of type GT26) from the Applicant, at full load, the difference between the non-preheated fuel at 15° C. and the fuel preheated to >100° C. leads to an increase in the efficiency of the combined cycle plant with a small drop in output power. The drop in output power arises from the fact that the hot water for preheating the fuel is extracted from the water/steam circuit.
At present, natural gas as fuel is preheated to >100° C. The maximum temperature is determined primarily by the capability of the components of the fuel system to withstand higher temperatures, for example at the controlling valves with non-metallic seals. It is however sought to further increase the fuel temperature.
The preheating of the fuel is basically known from the prior art. For example, the teaching of EP 0 095 555 concerns preheating and/or evaporating the fuel in oil burners in order to improve the combustion process. U.S. Pat. No. 6,233,914 discloses a method for the thermal pretreatment of a fuel in which solar steam is injected, for reformation, into the fuel. It is known from EP 0 903 484 to preheat the fuel in a heat exchanger by means of hot gas from the gas turbine plant. U.S. Pat. No. 6,269,626 or DE19512466 present similar systems for combined cycle power plants.
Furthermore, in GB2449181 it is proposed to use solar heat for the purpose of heating the combustion air in combined cycle power plants.
Even though such technologies have been proven, some disadvantages remain:                Normally, high-value heat at temperatures of up to >200° C. is used for preheating the fuel. Said heat could however be more effectively used in the steam turbine for increasing the performance and efficiency of the plant.        If the gas turbine is operated only independently in a single circuit, no heat for preheating the fuel is available from the water/steam circuit.        